A Modular Multilevel power Converter (MMC), also known as Chain-Link Converter (CLC), comprises converter branches each with a plurality of e.g. ten to forty converter cells, or converter sub-modules, connected in series, wherein the converter branches in turn may be arranged in a wye/star, delta, and/or indirect converter topology. A converter cell is either a bipolar cell with a full-bridge circuit or a unipolar cell with a half-bridge circuit, and comprises a capacitor for storing energy and power semiconductor switches such as insulated gate bipolar transistor (IGBT) devices, integrated gate-commutated thyristor (IGCT) devices, gate-turn-off thyristor (GTO) devices, or MOSFETs for connecting the capacitor to the converter branch with one or two polarities. The voltage per converter cell capacitor may be between 1 kV and 6 kV; whereas the voltage of a converter branch may be in a range from 10 kV to several 100 kV. An MMC controller with a processor and corresponding software, and/or with a Field Programmable Gate Array (FPGA), is responsible for controlling the converter cells and operating the power semiconductor switches by means of a dedicated (pulse-width) modulation scheme.
MMCs may be used in electric power transmission systems as ac-only Static VAR Compensators (Statcoms) and/or Flexible AC Transmission Systems (FACTS) devices for static power-factor correction as well as for voltage quality and stability purposes. A Statcom provides reactive power support to an electric power transmission network or grid to which the Statcom is connected, by producing or absorbing reactive power.
An operating MMC requires a certain amount of energy which must be provided to the converter before connecting the converter to an electric grid. To that purpose, charging, or pre-charging, of the converter cell capacitors is performed by way of passive charging or by reverting to external charging control.
Passive charging is executed by connecting the uncharged converter with blocked firing pulses to the main electric grid via charging resistors. The charging resistors limit the inrush current as the main breaker closes and the cell capacitors are charged to about nominal voltage by the grid voltage rectification through the freewheeling diodes of the converter. Passive charging of the capacitors is performed slowly and hence takes between ten seconds to several minutes to complete. No voltage balancing is required since the impedance of the cell DC capacitors is dominant and thus the voltage drift is minor in this time range.
External charging control on the other hand requires additional control hardware and auxiliary power supply. The uncharged converter is connected with blocked firing pulses to the auxiliary power supply to receive a charging voltage comparable to the grid voltage of the main electric grid. External charging preferably involves a low voltage auxiliary power supply connected to a dedicated step-up charging transformer transforming the low voltage of the auxiliary power supply to the charging voltage. Charging resistors are not required in this case since the charging transformers impedance limits the inrush current.
For most converter applications a standby operation or state is defined, in which the main breaker is closed and thus a main AC voltage is applied to the corresponding converter terminals, but the converter is not supposed to feed any current into the grid. The converter system in standby mode is ready to resume operation immediately upon a respective command from the control system.
For classical two and three level Voltage Source Inverters (VSI) the standby mode is implemented by blocking simultaneously all firing pulses, or gate pulses, directed to the semiconductor switches, which results in a stable state of the grid-connected VSI. The voltage of the DC link of a VSI is kept at about nominal by the grid voltage rectification through the freewheeling diodes of the converter, with a possible deviation between the voltages of the upper and lower capacitor of a three level VSI generally being a minor issue.
For MMCs a standby mode may likewise be implemented by blocking the firing pulses. However, for this type of converters such a mode of operation is not stable and may typically be active only for a few minutes before a deviation between, or among, the DC voltages of the cell capacitors reaches unacceptable values. Therefore the standby mode for MMC converters is conventionally implemented with all semiconductor switches actively switching and enabling a current to flow through the branches of the converter. This current allows to balance the individual cell capacitor DC voltages. The currents through the individual branches are selected such that cancelation is obtained at the connection points of different branches and thus no current is fed into the mains. Often the term ‘circulating currents’ is used to denote such a concept.
The standby mode using circulating currents to balance the individual cell capacitor DC voltages generates losses. These are no-load losses since the converter is not supposed to feed any active nor reactive power into the mains in standby mode. For applications such as Statcoms or rail interties with static frequency conversion for traction supply, which make extensive use of the standby mode, these stand-by losses result in substantial cost for the utilities and/or penalties for the converter supplier.